1. Field of the Invention
This invention is concerned with the technique for manufacturing the glass body having distribution of ion concentration, and is also concerned particularly with the ion exchange processing method, which is useful for manufacturing large-diameter gradient index lens.
2. Description of the Prior Art
In general, techniques for causing diffusion of specific ions into the glass body, thereby changing properties of the glass body by distribution of such ions are used in manufacturing of gradient index lens, optical waveguide, chemically reinforced glass, etc.
As means of causing diffusion of ions into the glass body as described above, the ion exchange processing method is the simplest, where the medium, such as molten salt, etc., which contains monovalent cations, is made contact with the glass surface and monovalent cations in the glass are exchanged with cations in the above medium.
For example, it is possible to manufacture a gradient index lens, having a distribution of refractive index within the section, which gradually decreases from the center towards the periphery, by having rod-like body fabricated by glass, which contains ion-exchangeable cations, such as Tl, Cs, Li, etc. and also has relatively high refractive index; by immersing this glass rod in molten salt, containing ions which contribute toward reducing the refractive index of the glass, such as Na, K, etc., and by exchanging the above ions in molten salt with ions in the glass.
In the ion exchange processing as described above, diffusion of monovalent cations in the glass is rate-determined. Normally, diffusion rate of these ions is very slow. For example, a long processing time of approximately 10 days would be required to obtain an ion exchange layer of 1 mm or so. This is the greatest disadvantage of this method.
However, since the ion diffusion rate will increase exponentially with temperature, it becomes possible to influence ion exchange in locations deep from the glass body surface in a short time, when the ion exchange processing temperature is increased.
However, when processing temperature becomes high, the viscosity of glass will decrease, thereby causing deformation. For this reason, in the conventional method it was not possible to conduct ion exchange processing at or above the temperature where viscosity falls below 10.sup.10 to 10.sup.11 poise.
Because of these factors, gradient index lenses manufactured using the ion exchange method were of small diameters approximately 3 mm at the maximum. Therefore, it was extremely difficult to mass-produce gradient index lenses of large diameters at low cost using conventional method.